I noticed in the episode "The One With The Blind Dates" that Joey got angry at Chandler and Monica for having sex with Emma in the house.

When Monica and Chandler asked how he knew he said he saw Chandler's head, and that he should get on top for once.

However in the scene where Joey realises they are having sex, he doesn't even open up the door to their bedroom and hears them only.

That is a pretty big mistake if you ask me.

I noticed that too! Maybe the editors just cut out Joey opening the door to save time, but that kinda drove me crazy!

u know joey he's strange about those things

No, he said and Chandler, your hair is messy, can't you stay on top for once? So that's how he knew!